Interpretive Summary: Agricultural practices can have major impacts on sediment loading, nutrient additions and water temperature in free-flowing streams in the Interior Pacific Northwest. We reviewed literature pertaining to agricultural impacts on water quality of Pacific Northwest streams, presented a case study examining the influence of flood irrigation on stream temperature, and used a novel framework to discuss implications of variability of water quality in space and time. Our findings and review suggest that stream water quality and the effects of agriculture on water quality are complex problems that vary strongly in both space and time. Contemporary policies and paradigms focus attention on discreet temperature (or other water quality) values, thus creating tension between managers and regulators, however, incorporating spatial and temporal variation into the concept of water quality will allow for a more representative characterization of the water quality regime and promote a more comprehensive understanding of the potential impacts of water temperature on the stream ecosystem and its inhabitants.

Technical Abstract:
European arrival to the Pacific Northwest (U.S.A.) in the late 1800’s signalled the beginning of an era of change for watershed use and water quality of free-flowing stream systems. Historic uses centered around trapping of beaver, utilization of livestock forage, and harvesting of anadramous fish and timber resources (Beschta, 2000). Impacts of these land uses on stream systems were severe; impaired flood plain development with beaver removal, interrupted nutrient cycling with overharvest of anadramous fish, streambank degradation due to overgrazing, and severe flooding associated with abusive logging practices (Trefethen, 1985; Meehan, 1991; Beschta, 2000). Perhaps most dramatic was the conversion of ecosystem type from stream system to impoundment associated with dam construction for hydroelectric power production (Beschta, 2000). The US Clean Water Act (CWA) of 1965 was crafted and passed into law in response to these and other issues, providing a legal and regulatory framework for managing land use practices that impact water quality (Adams 2007). Subsequent amendments of the CWA in 1972 and 1977 helped expand this framework to include impacts of non-point source pollution and mandated that states develop water protection programs (National Research Council, 1995; Adams, 2007).
While the forgoing issues continue to attract popular and political attention, legislative and regulatory mandates have dramatically reduced the acute effects of these practices and attention has now shifted to less dramatic, but nonetheless important associations between topical land use practices and water quality. There is an established and growing awareness of the impact of agriculturally-related non-point source pollution on stream systems. In this chapter we will 1) examine the scope of non-point source pollutant issues relating to the water quality/agriculture interface for streams in the Interior Pacific Northwest (PNW), 2) present a case-study examining the relationship between flood irrigation and in-stream temperature, and 3) make the case that stream temperature and non-point source water quality issues are complex problems that are best addressed by considering variation of the problem in both time and space.